The present invention relates to a method of joining two materials by mechanical interlock means therebetween and to the resulting article of manufacture. And, more particularly, the invention relates to an improved method of anchoring a polymeric liner or covering to an article such as a pipe fitting, pump part, small tank, valve part, or the like.
Various organic high performance polymeric materials, such as for example tetrafluorethylene (TFE) fluoro polymer, perfluoroalkoxy (PFA) fluoropolymer, and polyolefins, provide such desirable characteristics as high tensile strength, high flex strength, corrosion and wear resistance, low friction, and toughness. However, these materials often lack sufficient rigidity and structural integrity for specific applications unless anchored to a backing of metal, fiber reinforced thermosetting polymer, or other rigid structural material. Metal castings lined with such fluoropolymers provide corrosion resistant parts particularly suitable for service with corrosive chemicals. However, due to the poor bondability of such fluoropolymers, anchoring of a liner or other fluoropolymer component is particularly difficult.
A specific example illustrating the difficulties in anchoring such a liner to a backing of rigid material is that of lining a centrifugal pump casing with a layer of PFA fluoropolymer. The PFA fluoropolymer liner provides excellent corrosion resistance particularly suited for use in pipelines transporting highly corrosive chemicals.
There are several problems with previously available PFA fluoropolymer lined metal pump casings. One cause of such problems is the high shrinkage of the PFA fluoropolymer as it solidifies and cools from its high melt temperature range of 600.degree. to 700.degree. F. during the manufacturing process. The liner is formed over the suction and discharge flanges of the pump and is otherwise restricted from freely shrinking as it cools. This shrinkage creates residual stresses in the liner which cannot be removed and causes the liner to pull away from the metal casing of the pump, thereby reducing dimensional stability. This loosening of the PFA fluoropolymer liner also results in flexing of the liner during pump operation in response to variations of pressure caused by rotation of the pump impeller, potentially causing tearing or cracking.
Attempts to solve these problems by producing a free liner of PFA fluoropolymer and inserting it into a metal pump casing generally result in a poor fit. Furthermore, the PFA fluoropolymer liner cannot be adequately bonded to the metal casing of the pump because of the poor bondability of the fluoropolymer. Attempts to adhere the PFA fluoropolymer liner to the metal casing during the cool-down step of manufacture, either by relying on primer coatings or by adding mechanical locking features such as grooves or welded-in-place perforated metal, result in residual stresses as the liner shrinks away from these restraints. In addition, the adhesion of the liner to the metal casing, with or without the use of a primer, is lost upon subsequent permeation of moisture through the liner during service of the pump. The residual stress and subsequent loosening and flexing of the liner can result in tearing or stress cracking of the liner.
To overcome the problems associated with lining a metal pump casing with PFA fluoropolymer, the inventor has experimented with molding a pump casing of a fiber reinforced thermosetting polymer around a PFA fluoropolymer liner. After thoroughly stress-relieving the PFA fluoropolymer liner by thermal treatment and slow cooling in the free state, the liner has been preheated to the compression mold temperature of the fiber reinforced thermosetting polymer, generally within the range of 250.degree. to 300.degree. F., and subsequently inserted into the mold for forming the casing. A suitable fiber reinforced thermosetting polymer, such as glass fiber reinforced epoxy molding compound, was then molded around the preheated liner.
With this improved technique, the much reduced thermal shrinkage of the PFA fluoropolymer liner when cooling from this relatively low temperature and the reduced differential shrinkage between the liner and the fiber reinforced thermosetting polymer of the pump casing, compared to earlier techniques using a metal pump casing, result in much lower residual stress in the PFA fluoropolymer liner. A pump part produced in this manner provides an improvement over a pump part having a liner directly molded in a metal casing, but it still has some deficiencies. Even though differential shrinkage is reduced, there is still some loosening of the liner from the casing during use. In addition, during pump operation, negative pressure in the suction area of the pump may cause problems with liner flexing and potential tearing or stress cracking.
Another technique for manufacturing a lined structure utilizes polymeric sheets with partially embedded glass fibers protruding from one side. The polymeric sheets are assembled into simple shapes with the glass fibers protruding outward, and a rigid casing is formed over the protruding glass fibers to form a structure having a bonded liner. The glass fibers primarily reinforce the polymeric sheets and, secondarily, provide an interlock between the casing and the polymeric sheets. However, this technique cannot be used when complex shapes are to be formed.
The same difficulties of bonding or anchoring a PFA fluoropolymer, illustrated above with respect to the lining of a fiber reinforced thermosetting polymer pump part, are also encountered with other liner materials, with other armor materials, and with other articles such as pipe fittings, strainer parts, valve parts, small tanks, etc. More generally, the difficulties encountered are those of bonding or anchoring a fluoropolymer, or other organic high performance polymeric material having poor bondability, to another material.
Accordingly, it is an object of the present invention to provide an improved method of joining two materials, one of which is a fluoropolymer or other organic high performance polymeric material having relatively poor bondability.
It is a further object of the present invention to provide an improved method for anchoring a liner or covering to a structure such as, for example, a pipe fitting, pump casing, pump impeller, small tank, valve body, valve plug or the like.
It is still a further object of the present invention to provide an article of manufacture having two materials, one of which being a fluoropolymer or other organic high performance polymeric material, and mechanical interlock means therebetween such that the two materials remain tightly anchored together without the need for a chemical bond therebetween.